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Ordovician Period

Many of the animals present in the Cambrian had no specialized respiratory structures, and they generally shared a common characteristic they were covered with epithelium that allowed for the passage of water directly from the sea into the body of the animal. For them the entire body was a gill. Examples include many soft worm-like forms, such as nemerteans and the many varieties of the jellyfish-sea anemone clan, the cnidarians. The cnidarians were and are composed of but two cell layers, and...

Because animals are obligated to harvest oxygen from the atmosphere or from a watery medium, they are affected by the varying concentrations of oxygen in air and water. There is no way that sufficient oxygen can be gained by eating food or drinking liquid with oxygen contained in it. And because animals are obligated to dispose of the waste products of respiration, chiefly carbon dioxide and water vapor, they are affected by the varying concentrations of carbon dioxide in air and water as well....

The levels of carbon dioxide and oxygen postulated for the Cambrian Explosion interval are shown in the figure beginning this chapter, taken from the Berner curve introduced in the preceding chapter. According to this curve, oxygen levels soon after the start of the Cambrian Explosion 544 million years ago were about 13 percent (compared to 21 percent today) but then fluctuated. During this time carbon dioxide levels were far higher than they are today, tens of times higher in fact, and such...

We have dusted off our time machine for a look at the early Ordovician. As in the previous chapter's voyage, this machine is part airplane and part submarine and entirely imaginary. What we might see, however, comes from two centuries of paleontological work. We sweep down over the land surface and notice immediately that it is greener. There are still no trees or even commonly rooted plants, but mosses have spread and among them are primitive vascular plants. There is also the occasional...

As we saw in Chapter 8 the first dinosaurs were the bipedal saurischians, and they soon spawned another group, the ornithischians. The ornithischians, which began as relatively small, carnivorous bipeds, quickly evolved into herbivores and stayed that way, with both quadrupeds and bipeds. But they remained a relatively small part of the terrestrial fauna during the first half of the Jurassic. The reason for their rarity relative to the number of saurischians is probably related to oxygen...

The biostratigraphy of the Permian and the Triassic a review of the classification and distribution of Permo-Triassic tetrapods. Palaentol. Afr. 21 15-44. Broom R. 1932. The Mammal-like Reptiles of South Africa and the Origin of Mammals. London Witherby. Erwin D. 1993. The Great Paleozoic Crisis Life and Death in the Permian. New York Columbia University Press. Erwin D. 1994. The Permo-Triassic extinction. Nature 367 231-236. Holser W, et al. 1989. A unique...

So far we have concentrated on the effects that varying levels of oxygen have on animals. But animal life is itself totally dependent on plant productivity, and we are sure that major perturbations in plant diversity and or abundance had effects on the animal food chains. Thus, we should look at the effects of varying oxygen levels on plants. Like animals, plants need oxygen for life. Oxygen is taken up within the cells during photorespiration. But the levels are far lower than those needed by...

Chapter 1 posed the iconic question of what determined the body plans that we observe in the fossil record and on Earth today. Evolution has created each of the body plans, but what specific factors were involved Today, we can observe natural selection in action among microbes and among organisms with short generation times. Organisms are adapted to their environments and undergo evolutionary change if changes in their environment, either physical or biotic, affect their survival rates....

Alas, the total extinction of dinosaurs 65 million years ago (unless birds are considered dinosaurs, now accepted by many) will forever make it impossible to answer many pressing questions about their biology. So it is natural that we try to answer these questions using their nearest living relatives, the birds and reptiles. After lung type and metabolic type, some of the most interesting questions relate to reproductive strategy, and this too can be examined in the light of changing oxygen...

This book is a reinterpretation of selected and important events and evolutionary breakthroughs during the past 550 million years, the time of animals, showing in chronological fashion why this author believes that it was the varied kinds of adaptation to varying oxygen levels that was the major stimulus to evolutionary change among the animal phyla. The evidence comes from both the history we have observed and what we know of animals living today. Following two introductory chapters, one...

Romer's Gap ended in the Carboniferous Period (split in two in America, where we call it the Mississippian and Pennsylvanian Periods), and its European name comes from the fact that the majority of coal now found on Earth dates from this time. During this time oxygen levels rose in spectacular fashion, and in the last intervals of the Carboniferous and continuing into the successive Permian Period, oxygen levels finally topped out at nearly 35 percent, creating a unique interval in Earth's...

The major hypothesis of this book is that changing atmospheric oxygen levels over the last 600 million years have caused significant evolutionary changes in animals. There has obviously been great change over this period of time, but are there times of greater and lesser evolutionary change and, if so, can these be correlated with oxygen levels Restated, can the hypothesis that changes in atmospheric oxygen content over time have spurred evolutionary development (to form new spe cies) and...

To explain the conditions, flora, and fauna of the Cambrian, imagine that we have journeyed back in time to Earth circa 522 million years ago. To really do this right we need not only a time machine but also a spaceship, in order to better see the position of the continents, for continental position and geological processes accruing from plate tectonic processes had a determining effect on subsequent biotic history. The first thing we notice as we pass over the land surface is that there is so...

The Jurassic and Cretaceous oceans would have been dangerous places to swim in. The major Triassic marine predators just increased in number in the Jurassic and added a new and efficient kind of fish- and cephalopod-eater, plesiosaurs. In the Cretaceous yet another kind of tetrapod predator appeared as well mosasaurs displaced plesiosaurs and ichthyosaurs as the top carnivores in the sea. Ammonites continued to flourish but evolved large numbers of uncoiled...

Having looked at the respiratory structures of the most populous Cambrian animals, it is time to take a look at a small and insignificant group of fossils found at Chengjiang and later at the Burgess Shale small, fish-shaped animals that ultimately gave rise to us. The most famous of these was named Pikaia, an animal featured at length in Steven Jay Gould's book about the Burgess Shale animals called Wonderful Life. The origin of our own phylum is, of course, of intense interest and has been...

We know that the disappearance of the dinosaurs unleashed a torrent of evolution, producing, in short order, many mammalian taxa. And for the first time, true mammals of larger size evolved. Thus, after the Cretaceous extinction, after the smoke had cleared and the dinosaurs were no longer around, large mammals did begin to appear. But it took a while. Santa Barbara paleontologist John Alroy has meticulously studied the sizes of mammals through time. Through dint of hard work in the library and...

For the next few million years, plate motions should continue in the current directions. The Atlantic Ocean will continue to widen, and the Pacific Ocean will continue to close. But then we can expect major changes. By 50 million years from now a world map would show fantastic differences from the present-day, and these continental positions are more completely described in my 2003 book coauthored with Don Brownlee, The Life and Death of Planet Earth. Perhaps most noticeable would be the loss...

Throughout this book, it is proposed that times of low oxygen in Earth's history stimulated new kinds of evolution but were also times of low diversity. During high-oxygen times, conversely, diversity is high but the rate of new species formation is low. This hypothesis is based on the proposal that low oxygen forces new experimentation in terms of body plan. This proposal is supported by a new comparison of oxygen levels with data on the rate of new taxon formation. Thus, we should expect to...

Here let's return to a subject raised in the last chapter, but one whose history is even more important in this chapter's interval of time than last's. Metabolism is the term used to characterize the acquisition and use of energy by organisms, and metabolic rate is the pace at which acquired energy is utilized. It dictates the amount of fuel consumed and the amount of heat generated. It takes work to conduct the activities of life, which include physical activity, food processing, and tissue...

Let's now turn our attention to the evolution of the first amphibians, the vertebrate group that first colonized the land, or partially did. The fossil record has given us a fair understanding of both the species involved in this transition and the time. A group of Devonian Period bony fish known as Rhipidistians appear to have been the ancestors of the first amphibians. These fish were dominant predators, and most or all appear to have been fresh water animals. This in itself is interesting...

One of the most enduring scientific debates of the past two decades has been about the metabolism of dinosaurs. Were they endotherms, ectotherms, or so massive that neither applied The arguments have gone back and forth, based on evidence as disparate as bone structure, oxygen isotopes from dinosaur fossils, and reputed predator-prey ratios. Here it is proposed that endothermy originated as an adaptation to low atmospheric oxygen. If that was the case, endothermy should have evolved in multiple...

The complex air sac-lung system found in birds had to have evolved from a reptilian, sac-like lung. Here is the pathway envisioned by Paul in his 2002 book. The first of the so-called archosaurs were the primitive late Permian through early Triassic reptilian group (that we have called diapsids), which would eventually give rise to crocodiles, dinosaurs, and birds. Examples of this group included the quadruped form Proterosuchus (described above as one of the earliest Triassic archo-saurs)....

What is called the Permian extinction is really a series of extinctions, beginning at the end of the Guadaloupian Stage of the Permian Period (about 254 million years ago), with a second and far more severe pulse at the Permian-Triassic boundary itself, dated at about 251 million years ago. This double extinction at the end of the Permian has been known for about a decade or more. But now we are finding that even between these larger events there were smaller extinction episodes as well. What...

The change in oxygen levels over time seems to have provoked major changes in evolution. What about one of the biggest of all changes and the most important to us Can any aspect of the changing atmosphere be interpreted in a new way so as to explain the evolution of our own species In one sense it can. Johns Hopkins paleontologist Steve Stanley has dubbed us Children of the Ice Ages, and many paleoanthro-pologists think that the rise of high intelligence and culture was a means of dealing with...

Our visit back to the Cambrian leads to an inescapable conclusion in sheer numbers of individuals and species described as diversity and in sheer numbers of different kinds of body plans described as disparity , the arthropods were the most successful of Cambrian animals. How much of this success was due to their principal body plan characteristic segmentation Segmented animals are the most diverse of all animals on the planet today, and most are arthropods. All arthropods, including the highly...

With this background on animal respiration and the geological oxygen record, we are now ready to tackle the history of animal life as recorded in the rock record. The next chapters will examine in more detail major episodes in the history of animal life, their relationship to oxygen levels, and morphological adaptations that appear to have been brought about in response to changing oxygen. Thus, each chapter will not only deal with diversification and extinction trends but will also feature as...

Oxygen is thus essential for animal life, and animal life consequently has evolved a host of structures for its acquisition. Because Earth has widely varying oxygen concentrations in different environments, some of the differences shown in the Appendix are related to living in different oxygen regimes. But we must take into account a new variable as well. Not only does oxygen concentration vary from place to place, it has also varied through time. It seems possible that an organism's type of...

In Chapter 8 we looked at the questions of dinosaur metabolism and respiration. The proposal there was that the first dinosaurs were of a kind of animal never seen and not alive today. Through upright posture and an evolving air sac system, they developed respiratory efficiency the amount of oxygen extracted from air per unit time, or per unit energy expended in breathing superior to any other then-extant animal. But these early forms may have lost or never gained in the first place endothermy,...

Ichthyostega, long thought to mark the appearance of the first land vertebrate, may have been far more fish-like than first thought, and the fact that it lost its gills is not evidence of a fully terrestrial habitat. Long and Gordon point out that today there is a large diversity over a hundred different kinds of air-breathing fish. Air breathing has evolved independently in as many as 68 kinds of these extant fish, showing how readily this adaptation can take place. Ichthyostega may not even...

In general, respiratory organs are broadly classified as gills evagina-tions , which are used in water, or lungs invaginations , which are used in air. Respiratory organs evolved for water usually will not work in air and vice versa. The densities of the two media are so different that vastly different structures must be employed. That no animal has ever evolved an organ that could work both in air and water equally well suggests that it just cannot be done with biological material. We'll need...

As we saw in Chapter 7, the Permian extinction nearly annihilated land life. The therapsids were hit hard. Much less is known about the diapsids, for at the end of the Permian they were a rare and little seen group in the areas, such as the Karoo or Russia, that have yielded rich deposits with abundant dicynodont therapsid faunas. In the Karoo at least, only small fragments of diapsids have come from our uppermost Permian study sections, although two skulls now being prepared as I write this...

The middle Triassic was a time of amazing disparity on land and in the sea. In the latter, new stocks of bivalved mollusks took the place of the many extinct brachiopods, while a great diversification of am-monoids and nautiloids refilled the oceans with active predators. Fully a quarter of all the ammonites that ever lived have been found in Triassic rocks a time interval that is only 10 percent of their total time of existence on Earth. The oceans filled with their kind, in shapes and...

While at first it seems that there were many kinds of dinosaur body shapes, in fact there were really but three bipeds, short-necked quadrupeds, and long-necked quadrupeds. All three shared a common characteristic with birds and mammals a fully upright rather than sprawling posture. Let's look at these shapes in the context of atmospheric oxygen and ambient temperature when they originated and when they thrived. Any history of life must come to grips with and explain the extreme gigantism of...

The Carboniferous oxygen peak would have had consequences in addition to gigantism. Oxygen is combustible, and the more there is the bigger the fire it facilitates fuel ignition, and the fuel in question was the huge and global forest of the Coal Age. The Carboniferous Period may have witnessed the largest forest fires ever to occur on Earth at least until the dinosaur-killing and forest-igniting Chicxulub asteroid strike the well known Cretaceous-Tertiary, or K T extinction event of 65 million...

The GEOCARBSULF model is a recent combination of the much earlier models for carbon dioxide GEOCARB and oxygen isotope mass balance model . It is a computer model that takes account of the many factors thought to influence atmospheric oxygen and carbon dioxide. A computer model such as GEOCARBSULF must take account of forcings, processes that affect the oxygen levels. Chief among these are the rate of metamorphic and volcanic degassing of reduced carbon- and sulfur-containing gases, the rate of...

A respiratory system can be defined as an assemblage of integrated cells and organ and tissue structures that deliver oxygen to the various cells in an animal's body and then remove carbon dioxide from the body. Respiratory systems are highly variable across the animal kingdom, incorporating specific morphologies involved in gas exchange, such as our own lungs and the gills of fish, but gas exchange is just the first part of the respiratory cycle. Oxygen has to be transported to the cells, and...

There can be no discussion about the atmosphere without including the global ocean. The two are coupled even small changes in the temperature or chemistry of the global ocean can produce enormous changes in the atmosphere. The composition of Earth's present-day atmosphere is basically known. Essentially it is made up of two gases 78 percent of its volume is nitrogen, and 21 percent is oxygen. The remaining 1 percent is made up of trace amounts of other gases. Yet even at this small volume, this...

Here is what we might see were we to return to the world of 300 million years ago, when oxygen was at its highest in Earth's history. The first noticeable characteristic is the color of the sky. It is a polluted yellow-brown, irrespective of weather only in high winds does the air clear and then soon it muddies again. This is due to smoke from giant fires perpetually raging and new ones set alight with each lightning strike hitting the extensive forests of the temperate and tropical regions....